In a digital communication system, digital data is converted to analog signals by modulating the data onto a carrier. The modulated carrier is then transmitted over a physical medium, such as copper wires or a wireless radio frequency (RF) connection. The physical medium may be divided into bands, wherein each band is assigned to one or more carriers, tones, or sub-carriers. The amount of data that is allocated to a specific bandwidth depends upon the ratio between the signal power associated with the carrier and the noise power in that band. This parameter is referred to as Signal-to-Noise Ratio (SNR).
In order to achieve good performance in channels with high noise levels, the reliability of the system may be increased using techniques such as modulation, coding, or assigning an SNR margin. SNR margin is the additional SNR available for a communication channel after modulating the carrier with a data signal. For example, if a channel has a SNR of 15 dB, but is allotted 10 dB of data, the additional 5 dB is referred to as SNR margin or SNRM. SNR margin is a measure of a communications system's immunity to noise. Increasing the amount of data, while maintaining the same bandwidth results in a smaller SNR margin. As a result, the system can tolerate less noise before bit errors begin to occur.
SNR margin is used to mitigate the effects of crosstalk and other signal impairments that occur during transmission. SNR margin may be used, for example, for noise mitigation in multi-carrier systems, such as Asymmetric Digital Subscriber Line (ADSL) or Very high speed Digital Subscriber Line (VDSL) systems, which use Discrete Multi-Tone (DMT) modulation. Using SNR margin in a communication system presents a trade-off between data-rate efficiency and noise mitigation. In a multi-carrier system, the allocation of bits on each sub-carrier depends on the SNR available for that sub-carrier. Often, for the sake of link stability, the total available SNR is not used for bit loading. Instead, only a portion of the SNR is used for bit loading, and the remaining SNR on the sub-carrier is used to mitigate impairments. The unused SNR is the SNR margin. In one embodiment, 3 dB corresponds to one bit of data. Accordingly, for each 3 dB of SNR margin, one less bit of data is being transmitted on that channel.